This invention is related to a method and apparatus for depositing silicon nitride films.
Silicon nitride films having thicknesses between 50-1000 .ANG. are used as capacitor dielectrics, selective oxidation masks and for passivation in integrated circuits. These films are generally deposited by chemical vapor deposition ("CVD") in multiple wafer batch reactors that typically process batches of 50-100 wafers at a time. Typical CVD processes are described in U.S. Pat. No. 3,637,423 to Sestrich, et al.; U.S. Pat. No. 4,442,589 to Doo, et al.; U.S. Pat. No. 4,158,717 to Nelson, et al.; U.S. Pat. No. 4,181,751 to Hall, et al.; and U.S. Pat. No. 4,279,947 to Goldman, et al.--all of which are incorporated herein by this reference.
However, existing multiple wafer reactors and processes for the deposition of silicon nitride have several problems. One problem is that these reactors and processes often deposit a nonuniform silicon nitride film. The thickness and the electrical properties of the film vary across the diameter of each wafer and can also vary from wafer to wafer. This non-uniformity is even more severe for wafers having large diameters (exceeding 8") which are increasingly being used in the semiconductor industry. Processing of large diameter wafers in multiple batch reactors is risky because the entire stack of 50-100 wafers, each wafer typically worth between $500 to $5,000, is at risk.
Another problem with existing silicon nitride processes and reactors is that the wafers are typically exposed to ambient air during the loading and unloading of the wafers from the reactors. Exposure to air and moisture can cause an oxide layer to form on the wafers which inhibits the deposition of a silicon nitride film. Consequently, the oxide layer is typically removed before deposition of the silicon nitride film. This adds a process step and increases the cost of processing the wafer.
Reactors that process a single wafer at a time generally deposit a more uniform film. However, a critical problem with known single wafer reactors and processes is that condensates deposit in the gas conduits and on the reactor walls. These deposits cause the conduits to clog up and require that the reactor be frequently cleaned. Furthermore, the deposits on the reactor walls can eventually fragment and generate particles which contaminate the wafer. It is only in the final stages of the manufacturing process, when the fully processed wafers are each worth between $5,000 to $50,000, that the contamination is discovered, and the entire wafer must be scrapped. Thus, condensate deposits in the gas conduits and on the reactor chamber walls are undesirable.
Therefore, there is a need for a more reliable deposition process and deposition apparatus that can be used to deposit uniform silicon nitride films on large sized wafers without contaminant deposition or oxide formation on the wafer surface.